The present invention relates to a method and a system for analyzing a semiconductor fabrication process. More particularly, the present invention relates to a method and a system for analyzing a thermal diffusion process within semiconductor fabrication.
Chemical vapor deposition (CVD) is a process for forming thin film layers within the semiconductor integrated circuit fabrication. Usually, it includes atmospheric pressure and low pressure chemical vapor deposition (APCVD and LPCVD). By the process, silicon nitride, silicon oxide and polysilicon layers are formed on a silicon wafer substrate. In LPCVD, the process is carried out at a pressure below 100 Torr. Due to the low pressure, the thin film layers have good qualities and better step coverage for semiconductor fabrication. Typically, wafers are placed in a vertical or horizontal furnace where they are heated and brought to a low pressure state. A reaction gas is introduced into the furnace, and reaction material is deposited on the substrate either by reaction or by thermal decomposition of the reaction gas.
FIG. 1 is a schematic cross-sectional view of a conventional vertical LPCVD furnace. As shown in FIG. 1, the LPCVD furnace is a hot-wall reaction type tool for thin film deposition. The reactor is a quartz tube 102. Outside and surrounding the quartz tube 102 are a group of heating elements 108 for heating up the furnace. Since the group of heating elements 108 have five sections, the group of heating elements 108 is also called a five-zone heater. A vacuum pump P removes gas from the reactor. Reaction gases from outside pass into the reaction chamber through a gas injector 110. The wafers waiting for thin film deposition are loaded on a quartz boat 104. By this tool, a thin film layer, such as silicon nitride, silicon oxide and polysilicon or the like is deposited on the wafers.
Generally, the chemical deposition rate depends on temperature, pressure or flow rates of reaction gases. It is difficult to produce a thin film layer having a uniform thickness on the wafers from the bottom to the top of the furnace. For the five-zone heater, the furnace is divided into three regions, top, center and bottom. Usually different regions will form thin film layers with different thicknesses even though the thickness to be formed ideally is same. As a result, adjusting the temperatures for each heating element to form a thin film layer having uniform thickness is required. However, the adjustments of temperature are very individualized and cannot be determined by a simple rule. For example, when the heating element 120 is adjusted to enhance or inhibit the reaction at the bottom region, the heating elements 118 and 122 are adjusted as well. Because of the unpredictable reaction in the furnace, sometimes the heating element 114 or 116 affects the deposition reaction at the bottom region more serious than the heating element 118 or 122.
Accordingly, it is desirable to propose a method and a system to control the temperature adjustments for each heating element in order to form a thin film layer with a uniform thickness.